This invention relates to a secondary battery, more particularly to a secondary battery which has good charge/discharge cycle characteristics, is capable of being charged/discharged with a heavy current and has high energy density.
A secondary battery in which a main component of a positive electrode is a chalcogen compound of a transition metal such as TiS.sub.2, MoS.sub.2, etc., and a negative electrode comprising Li or an alkali metal mainly composed of Li has a high energy density, and efforts to develop it as a commercial product have been made.
A secondary battery in which a conductive polymer such as polyacetylene, etc. is used as a positive electrode and Li or an alkali metal mainly composed of Li is used as a negative electrode material has also been studied.
However, in such secondary batteries, the following problems occur requiring improvements in the batteries for practical use.
A first problem is caused by the fact that the negative electrode material is itself a Li foil or a foil made of an alkali metal mainly composed of Li. More specifically, during discharging of the battery, Li migrates as Li ions from the negative electrode to the electrolyte, while during charging, the Li ions are electrodeposited as metallic Li again on the negative electrode, and by repeating the charging and discharging cycle, metallic Li electrodeposited as accompanied therewith becomes shaped in dendrite. The dendrite-shaped Li is an extremely active substance, and decomposes the electrolyte to result in deterioration of the charging and discharging cycle characteristic of the battery. Further, as the growth of such dendrite, finally, the metallic Li electrodeposited product shaped in dendrite will penetrate through the separator to reach the positive electrode, whereby a short circuit will occur. In other words, this is the problem that the charging and discharging life is short.
A second problem is due to the fact that the positive electrode comprises a metal chalcogen compound as the main component. More specifically, generally as the discharging depth in charging and discharging becomes deeper, deactivation of a metal chalcogen compound will proceed rapidly. As the result, the battery capacity will be lowered to great extent after several charging and discharging cycles, whereby the battery can no longer be used in a practical application.
Among them, in order to avoid the first problem, it has been attempted that for constitution of the negative electrode, a carbonaceous material obtained by calcination of an organic compound is used as the carrier, and lithium or alkali metals composed mainly of lithium is carried thereon.
By using such negative electrode, precipitation of Li dendrite can be prevented, but on the other hand, the battery having the negative electrode assembled therein has a far smaller discharging capacity as compared with a primary battery of the same size, and also the magnitude of self-discharging has not been necessarily reduced to a satisfactory extent.
Also, for solving the second problem, it has been proposed to use a solid solution of V.sub.2 O.sub.5 and P.sub.2 O.sub.5 as the active substance for the positive electrode (see Japanese Provisional Patent Publication No. 134561/1984). However, the battery proposed here cannot be said to have solved completely the above problems, thus leaving the problems to remain as such.